1. Field of the Invention
The present invention relates to automatic focusing devices, and particularly to an improvement of an automatic focusing device of a video signal processing system in an image sensing apparatus such as a video camera.
2. Description of the Background Art
An image sensing apparatus such as a video camera is generally provided with an automatic focusing device for automatically focusing on an object. One operational principles of automatic focusing devices is the method of video signal processing system called mountain climbing control such as one recited in Japanese Patent Publication No. 39-5265.
In the mountain climbing control, a signal in a predetermined high frequency band is extracted from a luminance signal obtained from an image sensing element and an amplitude thereof is detected and converted into a level signal. A level signal thus obtained is subjected to an accumulating process for every predetermined period such as one field or one frame. The resulting value (hereinafter referred to as a focusing evaluation value) is determined for every field or frame.
When a position of a lens to be focused is close to a position of an in-focus state, a so-called edge portion of the obtained image is sharp. A signal forming such an image includes many high frequency components. On the other hand, in a defocused image, the edge portion is not sharp. Such a signal includes a relatively small amount of high frequency components. Accordingly, evaluation can be made as to whether a lens is at an in-focus position or not by detecting an amount of high frequency components in luminance signal components outputted by a video signal processing circuit. The above-mentioned focusing evaluation value is useful for evaluating the focusing state.
FIG. 5 is a diagram illustrating relation between a lens position and a focusing evaluation value. In the figure, the abscissa indicates positions of a lens, the ordinate indicates focusing evaluation values, and the curve 40 indicates characteristics of focusing evaluation values of high frequency components. The lens position P in the figure denotes a position of a lens when it is in a complete in-focus state.
As clearly seen from FIG. 5, the focusing evaluation value takes its maximum value at the lens position P. The focusing evaluation value increases or decreases at each of positions Q and R before and after the lens position P.
Accordingly, in mountain climbing control, while moving a lens in one direction, a focusing evaluation value is obtained for each field, for example. The obtained focusing evaluation value is compared with a focusing evaluation value one field before. As a result, if the focusing evaluation value in the present field is larger than the previous focusing evaluation value, the lens is further moved in the same direction as before. On the other hand, if the focusing evaluation value one field before is larger than the present focusing evaluation value, the lens is moved in the opposite direction. This is because, as seen from FIG. 5, a determination can be made that the position of lens has gone over the in-focus position P.
As described above, the lens is stopped at a position where a focusing evaluation value the same as a focusing evaluation value one field before can be obtained.
For example, suppose that a lens is at lens position Q of FIG. 5 in an initial state. A focusing evaluation value starts decreasing when the lens goes over the in-focus position P and reaches the position R by the abovedescribed mountain climbing control. Accordingly, the lens is returned in a negative direction and stopped at the in-focus position P. By doing so, the lens automatically moves to a position of an in-focus state.
As shown in FIG. 6, an object is generally located on a central portion of a field 50 in image sensing. For focusing upon the object, the target of focusing is usually not the entirety of field 50 but a focusing control area 51 which is a part in the center.
Also, in the above-described mountain climbing control, a false peak may occur in the focusing evaluation value due to noise or so. Accordingly, control is generally performed by checking whether a plurality of focusing evaluation values are continuously increasing, or decreasing.
FIG. 7 is a block diagram illustrating structure of one example of a video camera having a conventional automatic focusing device. In the figure, the video camera includes a lens system 2, a CCD (Charge Coupled Device) 3 provided on a light-receiving surface arranged at a position where an image of an object 30 is formed by lens system 2 for converting an image of the object into an electrical signal, a CDS (Correlation Double Sampling) circuit 4 for correlation-double-sampling a signal provided from CCD 3 as an output, a video signal processing circuit 5 for processing a signal sampled by CDS circuit 4 and outputs video signals (a luminance signal Yout, a chroma signal Cout), a motor 6 for moving a focusing lens included in lens system 2, and an automatic focusing device la for determining a direction of moving the focusing lens for focusing by processing a luminance signal Yout outputted by video signal processing circuit 5 to obtain a focusing evaluation value for controlling motor 6.
Video signal processing circuit 5 includes an AGC (Auto Gain Control) circuit 7 for amplifying a signal outputted by CDS circuit 4 to a predetermined level, a color separating circuit 8 for separating a signal outputted by AGC circuit 7 into a luminance signal and two color difference signals for outputting, a chroma processing circuit 9 for processing color difference signals outputted from color separating circuit 8 and outputting the same as a chroma signal Cout, and a luminance processing circuit 10 for receiving a luminance signal from color separating circuit 8 and outputting the same as an output luminance signal Yout.
Automatic focusing device 1a includes a synchronization separating circuit 11 for separating a vertical synchronization signal Vs and a horizontal synchronization signal Hs from a luminance signal Yout, a gate circuit 12 responsive to synchronization signals Vs and Hs provided from synchronization separating circuit 11 for passing only a signal corresponding to focusing controlled area 51 shown in FIG. 6, a BPF (bandpass filter) 13 for passing only high frequency components in a signal provided from gate circuit 12, a detecting circuit 16 for detecting a level of a signal outputted by BPF 13, a sampling circuit 24 for sampling a signal outputted by detecting circuit 16 at a predetermined frequency, an accumulating circuit 23 for obtaining a focusing evaluation value by accumulating a value outputted by sampling circuit 24 over one field, for example, and controlling circuit 21 for controlling movement of motor 6 so that a focusing lens is moved to an in-focus position on the basis of focusing evaluation values outputted by accumulating circuit 23.
In FIG. 7, a conventional video camera having such an automatic focusing device la operates as described below. Lens system 2 forms an image of object 30 on a light receiving plane of CCD 3. CCD 3 converts the formed image of object 30 into an electric signal by photoelectric conversion. CDS circuit 4 correlation-double-samples a signal outputted from CCD 3 and provides it to AGC circuit 7.
AGC circuit 7 amplifies a level of a signal outputted from CDS circuit 4 to a predetermined level and applies the same to color separating circuit 8. Color separating circuit 8 separates a signal amplified by AGC circuit 7 into color difference signals and a luminance signal and provides the color difference signals to chroma processing circuit 9 and the luminance signal to luminance processing circuit 10, respectively. Chroma processing circuit 9 processes color difference signals provided from color separating circuit 8 and externally outputs the same as a chroma signal Cout. Luminance processing circuit 10 processes a luminance signal applied from color separating circuit 8 and externally outputs the same as an output luminance signal Your. Luminance signal Yout is also applied to gate circuit 12 and synchronization separating circuit 11 of automatic focusing device 1a.
Synchronization separating circuit 11 separates a vertical synchronization signal Vs and a horizontal synchronization signal Hs from a luminance signal Your and provides the same to gate circuit 12. Gate circuit 12 applies a luminance signal to BPF 13 in synchronization with a vertical synchronization signal Vs, a horizontal synchronization signal Hs and a predetermined clock signal only when a position which is currently being scanned is inside focusing controlled area 51 shown in FIG. 6. BPF 13 extracts only predetermined high frequency band components in the luminance signal outputted from gate circuit 12 and provides the same to detecting circuit 16. Detecting circuit 16 detects a level of a signal of the provided high frequency band components and provides the same to sampling circuit 24. Sampling circuit 24 samples a level signal outputted from detecting circuit 16 at a predetermined frequency and provides the same to accumulating circuit 23. Accumulating circuit 23 accumulates a signal applied from sampling circuit 24 for one field and applies the same to control circuit 21. An accumulated value indicates a focusing evaluation value of that field. Control circuit 21 controls motor 6 on the basis of the focusing evaluation value provided from accumulating circuit 23 to move the focusing lens to an in-focus position by the above-described mountain clinching control. The automatic focusing process is thus performed by moving the focusing lens inside a lens system to an in-focus position using focusing evaluation values.
The above-described conventional automatic focusing device has the following problems. In order to avoid malfunctions due to noise or the like, a determination is made as to whether it is now in an increasing direction toward a peak or it has already gone over the peak by checking whether a plurality of focusing evaluation values are continuously increasing or decreasing. For example, if focusing evaluation values are continuously increasing twice or more, it is regarded that it is now in an increasing direction toward the peak and the focusing lens is moved in the same direction as before, and on the other hand, if they are continuously decreasing twice or more, a determination can be made that it has already gone over the peak, and the focusing leans is moved in the opposite direction.
However, if the focusing evaluation value continues varying alternately, as increasing and decreasing as shown in FIG. 3A, the above-described method can not make correct determinations. That is, as shown by the first through eighth focusing evaluation values in FIG. 3A, even through the focusing evaluation values are apparently becoming smaller on the whole, a determination that it has already passed the peak cannot be made. In this case, a determination is made that it has already gone over the peak only by seeing the seventh through ninth focusing evaluation values.
Furthermore, when taking an image of a moving object, for example, a focusing evaluation value may vary largely having no connection with a position of a focusing lens. That is to say, when a focusing evaluation value varies without any order as shown in the variation of the first through ninth values of FIG. 3B, a problem similar to the above-described one occurs. In such a case, since the reliability of focusing evaluation values is low, hunting may occur and the image will be out of focus in conventional apparatus.